One objective of a surveillance radar is to detect and locate a target, mobile or not. The determination of the position of a target is done notably by estimating its angular position which is defined by the angle between a reference axis and the axis linking the radar, or its carrier, to the target. For example, when the reference axis points towards the geographic north, the angular position of the target is defined by the azimuth angle. When the reference axis is the axis of the carrier, the angular position of the target is defined by the relative bearing angle.
The general principle of a surveillance radar consists in emitting a series of pulses via a steerable antenna which scans the angular space to detect the presence of a potential target. If a target is actually present in the beam of the antenna, the signal pulses are reflected on this target and return echoes to the radar which includes suitable reception means. Means for processing and analysing the received echoes are implemented in order to deduce therefrom information concerning the location of the target, notably its angular position, its distance relative to the radar or even its speed.
Three methods for analysing the received signal to determine the angular position of a target using a radar are notably known.
A first known method uses a single-channel radar which performs an analysis of the amplitude (or of the power) of the received signal during the scanning of the antenna. The amplitude of the signal received by the radar varies according to the azimuth of the antenna following the appearance of the antenna pattern. The azimuth of the detected target is obtained by searching for the maximum amplitude. This principle is illustrated in FIG. 1.
The efficiency of this method, and in particular the accuracy of the estimation of the azimuth of the target, is greatly dependent on the signal-to-noise ratio in reception. The lower this ratio, the more difficult it is to detect the target because the noise can generate spurious amplitude spikes.
Another known method consists in performing an analysis of the received signals in the frequency domain. This type of method is implemented by single-channel radars said to be working in “Doppler mode”. In such cases, the radar emits a series of pulses at different successive instants. The echoes received after reflection on the target are grouped together in blocks, a block comprising several pulses. For each pulse received, the azimuth, or the relative bearing, of the antenna is known to the radar, so an average azimuth is then defined for a block of pulses. By way of an analysis in the frequency domain of the pulses of a block, notably through a Fourier transform applied to this block, it is possible to detect the presence of the target by the level of the amplitude of the echoes received by the radar. Each block has an associated average azimuth of the antenna pointing direction. The angular locating of the target then consists in assigning, as estimate of the azimuth of the target, the average azimuth of the block for which a target is detected. The target may be detected on several consecutive blocks, a direction finding method can then be used in order to average the azimuth of the blocks by weighting them by the amplitude of the received signal.
This type of method has the drawback of limiting the azimuthal detection resolution to the angular resolution separating the average azimuth of two blocks and therefore indirectly to the size of a block. This drawback affects the efficiency of the detection with respect to the accuracy of the estimate of the angular position of the target. It is possible to remedy this drawback by implementing an azimuthal overlap between two or more consecutive blocks, but, the greater the rate of overlap, the more complex the calculation becomes, which affects the implementation cost.
A third known method consists in using a radar that has at least two reception channels. These two channels are commonly designated by the terms sum channel and delta channel, and the azimuth of the target is obtained by calculating the angle error measurement, for example taken to be equal to the scalar product of the signals obtained from each of the two channels. The angle error measurement value makes it possible to deduce therefrom the difference that there is between the angular position of the antenna and the real position of the target.
This third method presents the advantage of considerably increasing the angular location accuracy compared to the abovementioned second method, but it entails the use of a radar with two reception channels, a factor that increases the cost of the equipment.